1. Field of the Invention
The present invention relates to an improved method and apparatus for producing formed or molded parts from molding materials that do not flow freely, which will be referred to as compaction molding compounds. More particularly, the present invention pertains to such a method and apparatus which can be efficiently used to mold high quality shaped or non-planar parts from inexpensive compaction molding compounds.
2. Description of the Background Art
In the art, compaction molding compounds (or CMC), as well as apparatus (molds, presses, etc.) and methods used to compact such materials into boards and other planar or nearly planar objects, are known.
Compaction molding materials have been commonly prepared by mixing suitable solid material such as wood chips, fiber, grain hulls, straw, sawdust, papermill sludge (see, for example U.S. Pat. No. 4,303,019), etc. or mixtures thereof with a suitable binder or binder mixture of synthetic resins or the like. For wood chips and powders, urea-formaldehyde resin, melamine-formaldehyde resin, phenol-formaldehyde resin, polyvinyl acetate resin, casein, polymeric diphenyl methylene diisocyanate resin (PMDI) are the most commonly used binders, as reported by Terry Sellers, Jr., in Forest Products J. vol. 51, No. 6, p 12 (2001). In the case of rice hulls, which may be used in whole or size-reduced form, phenol-formaldehyde resin (see, for example, UK Patent 1403154 and U.S. Pat. No. 3,850,677), polyurethane foam stock liquid (see, for example, Japanese Patent Publication 58033445), urea resin adhesive (see, for example, Japanese Patent Publication 58033445), synthetic rubber latex (see, for example, Japanese Patent Publication 63118203), and a variety of thermoplastic binders have been used. Mixture of rice husks and wood pulp has also been reported using various binders (see, for example, Japanese Patent Publication 56015339). Such conventional mixtures may be in loose particulate form if simple flat press is to be used to produce flat particle boards.
To shorten the cycle time of molding, U.S. Pat. No. 5,078,938 introduces hot gas or steam into press during pressing cycle. U.S. Pat. No. 6,238,199 describes gas channels in the mold and blocking devices to control the flow of the hot gas.
It is very desirable that the major solid content of the CMC may be waste materials of various industries, and thus can be obtained for little or no cost, e.g., wood chips from the lumber industry, papermill sludge, grain hulls, etc. Rice hulls and other grain hulls are particularly desirable for use as the solid content because they typically are available from the rice and grain processing industries in clean form and near uniform size such that no further processing is normally required to remove undesired matter, etc., unlike wood chips which must be processed to remove bark, etc. nor to grind for size reduction. Also, rice and grain hulls naturally contain a significant amount of silicon which gives improved strength and resistance to moisture and insect to parts produced from the CMC.
To produce shaped articles other than flat panels, preformed mats such as the loosely felted mats made of wood flakes described in U.S. Pat. Nos. 4,061,813, 4,248,163, 4,408,544, etc. have conventionally been used. These matted CMCs are furnished in a mold or a simple two-platen press and are compacted to reduce or eliminate voids and increase the apparent density of the formed parts. Hardening takes place by heating under compression so that the furnish is solidified in compacted cavity geometry. The molded density of the formed part correlates strongly with developed structural strength. However, the part will fail if any portion of the part is compressed beyond the compressive strength limit of the CMC. Especially when high part density is desired to develop high strength, it is essential to control the part density uniformly and below the compressive failure point. As solids have very low solid compressibility, the failure point is close to the point of nearly complete elimination of voids. If any portion of the furnish reaches such a limit during the compacting cycle, the mold can not be closed further without destroying the part and/or the mold. Even if the part does not fail, uneven compaction will often result in unacceptable and unintended variation of part density. On the other hand, if any portion of the part is subject to draw or stretching deformation, failure by void formation or tear will be observed.
Due to the above-discussed considerations and restrictions on operating conditions, successful molding of CMCs has been limited to very simple part geometry which only requires very small deformation from flat sheet geometry of the preformed mat or initial furnish. For example, Canadian Patent Nos. 1154815, 1198314, 1246313, 1247880, 1256905 describe use of plant fiber, grain husk, straws, stalk, etc. to form biodegradable dishes. The geometry of dishes deviates very little from flat panels, requiring almost no draw during forming. Compaction using male and female dies in a simple press is sufficient to compact the furnish to produce the final shape. Similarly, U.S. Pat. No. 4,061,813 describes pressing a preformed mat to form a corrugated shape suitable for use as building material. In this case, the degree of draw may be somewhat higher than that needed for forming tableware but still remains relatively small. U.S. Pat. No. 4,131,664 describes compaction of composite mats to produce automotive acoustic panels, which also requires very small draw or deformation. Otherwise, the mat easily tears apart during compaction if the mat is pulled by the draw. Even if it does not tear, the draw decreases the density or forms voids, counteracting the desirable effect of compaction. Numerous patents, including U.S. Pat. Nos. 4,248,163, 4,408,544, 4,440,708, 4,790,966, and 4,960,553, have attempted to mitigate this damage from draw in forming shipping pallets. The disclosed designs are mostly flat or planar, but legs had to be drawn from planar preformed mats. These patents report such difficulty in producing structurally sound drawn legs that they had to invent many additional steps to form the legs, such as interrupting the molding process to add more furnish in the drawn area or forming the drawn area in a separate molding step before the planar section is over-molded. U.S. Pat. No. 4,073,851 teaches use of pourable particulate compaction molding compound to produce nearly constant thickness drawn walls by using a stepped die which restricts material pushed down and a matching wedge die which pushes the molding compound into a cavity, creating a draft angle. U.S. Pat. No. 4,078,030 teaches matching the die cavity thickness profile which will minimize or compensate the effect of draw. Such a technique, while it may be useful for certain cases, forces loss of design flexibility as the design is dictated and limitations put on it by the requirement of trying to counteract the effect of draw that creates density variation or void formation.
With the conventional practices described above, which use a simple two-platen press, some curved surfaces may be formed in one step from flat preformed mats or furnish, but the geometry of parts that can be formed thereby is very limited and the produced part has to be thinner in the drawn portion thereof. Molding parts with multiple thicknesses with uniform density and structural integrity is not possible with this conventional technology.
There have been few reported cases of using multiple-platens or pistons to compaction mold fully three-dimensional shapes. U.S. Pat. Nos. 4,459,194, 4,459,195 and 5,100,601 describe compaction molding I-beams, railroad ties and similar long elongated structural members with constant crosssection from a mixture of cellulosic particles of various length (pin chips), which are described as reacting to compaction force by superficial orientation and surface hardening. Vertically moving platens and horizontally moving platens are used to compact the four sides of the structural member profile. These movable walls are actuated sequentially and repeatedly, similar to repeated hammer blows on four sides, to bring about the superficial orientation and surface hardening. Applicability of the technology disclosed in these patents is limited to formation of elongated structural beams, and the forming operation is limited to the specific sequence and magnitude of platen movements described.
Otherwise, there are significant differences between the technology of the discussed patents using a simple two-platen press or variation thereof as described above and the technology according to the present invention as discussed herein. For example, the resulting part density distribution pattern achieved with the technology of the discussed patents is quite non-uniform and dependent upon the part geometry except when flat board of uniform thickness is produced, while the present invention produces uniform density if that is desired or produces controlled variable density if that is desired for a variety of complicated part geometries.
The goal of complicated platen movement in the listed patents is to introduce favorable particle fiber orientation on the part surface. This will be useful only when the compacting compound does not transmit compacting pressure uniformly through the part thickness, only creating superficial deformation and orientation near the surface, and is incapable of producing a uniform compaction condition such as is achieved by the present invention. Correspondingly, the compacting material suitable for use in the patented technology will not be suitable for use in forming compacted parts with the simple two platen press or according to the present invention, and vice versa. In addition, the device of the patents requires some movable platens or walls to move perpendicular to others, and the compacting movements are actuated in sequence one at a time, unlike the simple press or the present invention. When two or more platens move simultaneously in the method and apparatus of the listed patents, they only duplicate the movement of a simple press.
As will be understood from the foregoing, although various CMCs and apparatus and methods for processing same are known, the conventional apparatus and methods have significant limitations and restrictions associated therewith. A great need still exists in the art for a compaction molding apparatus and method which can be efficiently and economically used for forming a variety of part geometries with greater design freedom, geometries that have been previously considered impossible to mold in one molding cycle.